Light emitting devices and diodes are based on a forward biased p-n junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative incandescent and fluorescent lighting technologies. The efficiencies of LEDs can be quantified by three main factors, internal quantum efficiency, injection efficiency, and the extraction efficiency. The latter being the basis for the present invention. Several other factors affect the overall efficiency of solid-state lighting applications, including phosphor conversion efficiency and electrical driver efficiency. However, these are beyond the scope of the present invention.
One of the main limiting factors reducing the extraction efficiency in LEDs is the emitted photons being totally internally reflected and trapped in the high refractive index of the epi-material. These trapped waveguide-modes propagate in the LED structure until they are scattered, escape or reabsorbed. The thickness of the light emitting structure determines the number of modes that can be set up. Many methods have been successfully employed to improve light extraction in LED heterostructures. These include shaping LED die, as described in U.S. Pat. No. 6,015,719 and U.S. Pat. No. 6,323,063, flip-chip mounting of LEDs as described by Wierer et al. in Appl. Phys. Lett., 78, Pg. 3379, 2001, roughening of the top surface as taught by Schnitzer et al in Applied Physics Letters 63, 2174, 1993, and the use of omnidirectional reflectors as suggested by Fink et al. in Science vol. 282, Pg. 1679, 1998. Other methods suggested include the use of periodic texturing on at least one interface of the structure to improve light extraction out of the light emitting region, as described in U.S. Pat. No. 5,779,924.
To provide light emitting devices with high current and thermal driving capabilities, the vertical type n-p contact configuration in GaN material systems has also been adopted recently. Such examples have been disclosed in U.S. Pat. No. 6,884,646 and published U.S. Patent application 20060154389A1. However, one major drawback with such vertical type light emitting structures is the existence of optically lossy metal contacts in the close vicinity of the light emitting heterostructure. Trapped modes in the high index light emitting device typically undergo multiple internal reflections. The photons reflected at the interface between the metallic contact surface and the heterostructure material experiences large losses, thereby reducing the total light output of the light emitting diode.
Back Light Units (BLU) for LCD panels are key elements to the performance of an LCD panel. Currently, most LCD panels employ compact cathode fluorescent light (ccfl) sources. However, these suffer from several problems such as poor colour gamut, environmental recycling and manufacture issues, thickness and profile, high voltage requirements, poor thermal management, weight and high power consumption. In order to alleviate these problems LCD manufacturers are implementing LED BLU units. These offer benefits in improved light coupling, colour gamut, lower power consumption, thin profiles, low voltage requirements, good thermal management and low weight.
Another application for LED modules is in light engines for front and rear projectors. Conventional High Intensity Discharge (HID) type projector light engines have always been hindered by low efficiency and short lifetime resulting in slow adoption into consumer markets.
Thus, there are a wide range of applications for LED modules, if the problems limiting the efficiency can be alleviated. There is therefore a need for a more efficient design of LED, which can achieve the performance levels required for this type of solid state lighting device to replace more conventional sources.